The invention relates to a method of displaying a moving object against a fixed background on data display apparatus for displaying as an entity on the screen of a display device a quantity of data which is represented by digital codes stored in a display memory, the displayed data being in the form of discrete pixels or dots each of which has its colour and/or luminance defined by a respective digital code in the display memory at a location corresponding to the position of the pixel in the display, the apparatus including a processor for controlling digitally the storage, selection and display of data including background data.
The invention further relates to digitally operable data display apparatus of a type for displaying as an entity on the screen of a CRT (cathode ray tube) or other display device a quantity of data which is represented by digital codes stored in a display memory.
The displayed data can be, for example, a 320.times.250 resolution dot matrix colour display and in the case of raster scan display device the digital codes stored in the display memory are accessed repeatedly by the processor to update the display in a recurrent cycle of scanning lines which may be produced with or without interlaced field scanning.
A problem that is encountered with such bit-map displays, as they are termed, is to produce real-time movement (or animation) of an object in the display under logic processor control, because the time available for plotting the object in one position, erasing it, re-plotting the background at that object position and then re-plotting the object at a new position, is very small.
The cycle of logic operations which is required to move an object against a fixed background comprises:
(i) reading out from the relevant memory locations in the display memory the digital data for an area of background corresponding to a new position where the object is to be moved to and storing this data elsewhere to save it,
(ii) writing into the display memory at the vacated memory locations the digital data defining the shape of the object, with residual background, if any,
(iii) waiting at least one frame (refresh) period to allow the object to be displayed at the new position,
(iv) replacing the digital data for the area of background in its original memory locations in the display memory to `cancel` the object at the position,
(v) computing the next position for the object.
In existing data display apparatus, steps (i) and (iv) can involve a known technique of manipulating digital data for the smallest rectangular area of background which the object will fit into. Thus, if the object has an irregular shape, the digital data for more pixels than are strictly necessary has to be manipulated.
It is preferable to do this because the computing time which is necessary to perform the logic decision for deciding to copy or not a pixel is usually longer than the computing time actually required to perform the copy. However, existing systems cannot avoid performing this logic decision for step (ii) in order to enter any residual background in the rectangular space occupied by the object.